GH-113528: Move a few misplaced pathlib tests (#113527)

`PurePathBase` does not define `__eq__()`, and so we have no business checking path equality in `test_eq_common` and `test_equivalences`. The tests only pass at the moment because we define the test class's `__eq__()` for use elsewhere.

Also move `test_parse_path_common` into the main pathlib test suite. It exercises a private `_parse_path()` method that will be moved to `PurePath` soon.

Lastly move a couple more tests concerned with optimisations and path normalisation.